488 Genetic Theory and Evolution 



ways. In a crude way, this is what is actually visible when the salivary 

 chromosomes of Drosophila species of different taxonomic diversity 

 are compared, and there is no reason why the crude visible diversity 

 of pattern should not reflect the intimate diversity of the chemical 

 pattern of the genie material. Evolution, in this case, would not 

 require the origin of new genes, but only shifts in the hierarchy of 

 patterns and sub- or super-patterns, which may be small and affect 

 only minor characters, or may be large and affect major parts of the 

 organization. In this picture the chromosomes of Protozoa do not 

 have fewer genes but a simpler, less diversified and less hierarchical 

 pattern of the genie material, which may be essentially the same as in 

 all higher organisms. Evolution of the genie material, then, is internal 

 diversification, not addition of new atomistic units. 



Considerations like these have led me to postulate ( 1940 ) , as a 

 hypothesis, that scrambling and repatterning of the polarized se- 

 quences of chromosomal sections may occur occasionally in a single 

 event, which I called "systemic mutations." Such repatternings in all 

 grades, from small inversions or transpositions in one chromosome to 

 a complete repatterning of all chromosomes, may lead, if viable, to a 

 large over-all effect changing major features of development and 

 producing in one step ( or a few successive ones ) a major evolutionary 

 deviation. If the centromere region remains unchanged, synaptic 

 pairing in the heterozygote is still possible; and a homozygote can be 

 produced in time. As I said, this is a hypothesis only, and such a 

 happening may never be observable. But it is good to keep in mind, 

 when criticizing this hypothesis, that nobody has ever succeeded in 

 producing a new species, not to mention the higher categories, by 

 selection of micromutations. The critics of our hypothesis overlook 

 that it is derived directly from the undeniable facts of increasing 

 differences of intrachromosomal pattern with taxonomic distance, 

 where it can be observed, namely, in the salivary gland chromosomes 

 of Drosophila species. Either this is a product of chance and without 

 any meaning, or it has an evolutionary meaning. If the latter, it leads 

 logically to a hypothesis like that of systemic mutation (see Gold- 

 schmidt, 1940), This hypothesis involves, of course, the idea that 

 evolution, except on the lowest intraspecific level, proceeds by salta- 

 tions rather than by slow accumulation of small differences. The 

 reasons that compel me to give preference to this view have been 

 repeatedly given and do not belong to the present discussion. But 

 one aspect of the hypothesis of evolution by saltation is linked to the 

 theory of genie action and will therefore be discussed briefly. 



